Device for carrying out a heterogenously catalysed reaction and method for producing a catalyst

ABSTRACT

To carry out a heterogeneously catalysed reaction, a reaction mixture comprising hydrocarbon and water is fed onto a catalyst that is produced by compressing at least one catalyst powder into a highly compressed layer which forms a shaped body. The reaction mixture is pressed through the catalyst layer with a pressure drop.

BACKGROUND AND SUMMARY OF INVENTION

The present invention relates to a device for carrying out aheterogeneously catalysed reaction in which a suitable reaction mixtureis fed onto a catalyst, and to a process for producing a catalyst whichis suitable in particular for use in a device of this nature.

An example of a heterogeneously catalysed reaction is the generation ofhydrogen from hydrocarbon or alcohol, in particular methanol (methanolreforming), in which a reaction mixture comprising hydrocarbon oralcohol and water is fed onto a catalyst. Further examples are thereduction of carbon monoxide levels so that carbon dioxide is liberatedin a so-called hydrogen shift reaction, the oxidation of carbon monoxidein which a CO-containing gas and an O₂-containing gas are fed onto acatalyst and a combustible starting material is burnt with the additionof an O₂-containing gas in a catalytic burner.

Obtaining hydrogen from methanol is based on the overall reactionCH₃OH+H₂O→CO₂+3H₂. In practice, to carry out this reaction a reactionmixture comprising the hydrocarbon and steam is guided past a suitablecatalyst during heating, in order to produce the desired hydrogen in atwo-stage or multistage reaction sequence. A two-stage methanolreforming device of this nature is known from EP 0,687,648 A1. In theknown device, the reaction mixture is fed to a first reactor, in whichonly partial conversion of the methanol is desired. After it has flowedthrough the first reactor, the gas mixture, which still contains someunconverted starting materials, is guided to a second reactor which isconstructed optimally for the residual conversion. The reactors aredesigned as plate or bed reactors in which the catalyst is provided inthe form of a bed or a coating of the dispersion passages. Furthermore,catalysts in the form of coated metal sheets, lattices and foams throughwhich the reaction mixture flows are known.

EP 0,217,532 B1 has disclosed a process for the catalytic generation ofhydrogen from mixtures of methanol and oxygen using a gas-permeablecatalyst system in which a hydrogen generator is provided with an upperreaction zone and a lower reaction zone, the reaction mixture ofmethanol and oxygen being fed into the upper reaction zone. After it hasflowed through the upper reaction zone, the reaction mixture is guidedinto the lower reaction zone, in which, as a result of spontaneousinitiation of the oxidation of the methanol, the temperature rises tosuch an extent that partial oxidation of the methanol begins in theupper reaction zone in the presence of a copper catalyst and hydrogen isformed.

Working on the basis of this prior art, the invention is based on theobject of providing a device of the generic type which has as simple andcompact a structure as possible and in which the amount of catalystmaterial required for the conversion of a specific mass flow of fuel isminimized. A further object of the invention is to specify a process forproducing a catalyst which enables the said minimization of catalystmaterial and the simple and compact structure to be achieved.

To achieve this object, the invention proposes a device for carrying outa heterogeneously catalysed reaction. Consequently, the device accordingto the invention comprises a catalyst which is formed by compressingcatalyst material into a thin, large-area layer, it being possible topress the reaction mixture through the catalyst with a pressure drop. Incontrast to the known devices, such as hydrogen reactors and the like,the catalyst is not designed as a simple surface structure, around whichthe reaction mixture simply flows, but rather as a highly compressedthree-dimensional layer through which the reaction mixture is pressedwith considerable pressure applied. The result is a high utilization ofthe capacity of the active catalyst centres and a high reaction rate atthe centres. Due to the considerable pressure drop while the reactionmixture passes through the catalyst layer according to the invention,the flow resistances to the supply and removal of the starting materialsand products of the reaction do not play any major role, so that thesupply and removal of the substances involved in the reaction can be ofsimple form. The considerable compression of the catalyst materialproduces a highly compact catalyst layer, with the result that theproportion of the total volume and weight of the reactor which is formedby the gas space and solids which are not catalytically active (such asfor example metal support sheets and the like) is considerably reducedcompared to known devices. Preferably, the catalytic material used isfine-grained catalyst granules or powder. In this way, good mass andheat transfer to and from the inner areas of the catalyst grains isensured even at high reaction rates. Moreover, the proportion of poresthrough which the mixture can flow increases as the grain sizedecreases, i.e. the number of “blind alleys” for the gas flow decreases.Flowing through the layer imposes a high level of turbulence on thegases, with the result that the film diffusion resistances around thegrains of the catalyst material are reduced, leading to improved heattransfer through convection.

In one configuration of the invention, the catalyst layer is arrangedsubstantially at right angles to the direction of flow of the reactionmixture. The result is particularly short paths for the gas to flowthrough. Due to the large-area, highly compressed configuration of thecatalyst layer according to the invention, in the event of the gasesflowing at right angles, even a short distance is sufficient to achievea high level of reaction with a high pressure drop.

In a particularly advantageous configuration of the invention, thecatalyst material is compressed with a support structure, with theresult that the catalyst material is mechanically stabilized and/or theconduction of heat is improved. The support structure is advantageouslya three-dimensional lattice-like structure (matrix), which in a furtheradvantageous configuration of the invention is a metallic supportstructure. The metal used is, for example, copper, in particulardendritic copper.

In an advantageous configuration of the invention, the catalyst materialcontains a precious metal, in particular platinum. The added preciousmetal, which is preferably platinum, although the use of other preciousmetals is also possible, reacts even at relatively low operatingtemperatures and thus serves to heat the catalyst arrangement. Thismeasure significantly improves the cold-start performance of thecatalyst arrangement, which is advantageous in particular for use in themobile hydrogen generation sector.

In a particularly advantageous refinement of the invention, a pluralityof layers which are connected in parallel are provided. This allows thetotal surface area through which the reaction mixture is to flow to bespread over a plurality of layers which are arranged one behind theother but are connected in parallel. This “modular design” results in aparticularly compact structure of the catalyst arrangement.

To simplify the supply and removal of the substances involved in thereaction, in a further configuration of the invention passages forguiding starting materials of the reaction mixture and the reactionproducts are provided in the at least one catalyst layer.

In a further configuration of the invention, oxygen, which may promoteor be required for the reaction, is fed to the reaction mixture only atthe level of the at least one catalyst layer.

According to the invention, to produce a catalyst which can be used inparticular in a hydrogen generation device according to the invention, ahighly compressed layer which forms a shaped body is formed from atleast one catalyst powder by compression, the catalyst powder comprisingdendritic copper in powder form.

In one configuration of the invention, the shaped body is sinteredfollowing the compression, resulting in particularly high strength ofthe catalyst according to the invention.

In a further configuration of the invention, passages for guidingstarting materials and products of the catalytic reaction are formed inthe shaped body during compression. Advantageously, these passages areproduced by the introduction of spacer elements which can be removedagain in a subsequent process step. The spacer elements areadvantageously removed by being burnt, pyrolysed, dissolved orvaporized.

In a further advantageous configuration of the invention, a furtherpowder layer is pressed onto a ready-sintered shaped body and is thensintered. This allows a catalyst with a plurality of layers positionedone above the other to be produced in a type of sandwich structure in amultistage production process, which layers are connected in parallel bysuitable passages being formed. As a result, the total catalyst volumethrough which the reaction mixture is to flow can be spread over asmaller cross-sectional area while nevertheless maintaining the conceptof the high pressure drop over a short flow path.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is diagrammatically illustrated with reference toexemplary embodiments in the drawing and is described in detail belowwith reference to the drawing, in which:

FIG. 1 shows a highly diagrammatic illustration of the way in which acatalyst layer according to the invention functions.

FIG. 2 shows a perspective illustration of a stacked arrangement,according to the invention, of catalyst layers connected in parallel.

FIG. 3 shows a perspective illustration of the further exemplaryembodiment of a single catalyst layer according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagrammatic side view of a catalyst layer 10 accordingto the invention which is formed by compression of catalyst material toform a thin, large-area, highly compressed layer. The layer 10 forms ashaped body with a thickness d which is, for example, 1 mm. The catalystmaterial used is a fine-grained catalyst powder or granules, the grainsof which have a diameter of approx. 0.5 mm or less. The compressiontakes place, for example, at temperatures of approx. 200° C. to 500° C.

The catalyst layer 10 illustrated forms part of a hydrogen generationdevice (not shown in more detail) in which the starting materials of thereaction mixture are fed onto the catalyst layer 10 under pressure,substantially at right angles to the said layer, and are pressed throughthe said layer. As it flows through the catalyst layer 10, the reactionmixture undergoes a pressure drop Δp of approx. 100 mbar or more (forexample 1 to 4 bar). On the opposite side of the catalyst layer 10, thecatalytic reaction products emerge in the direction indicated by thearrow.

To make the catalyst material more mechanically stable and/or thermallyconductive, the catalyst material is pressed into a support structure.This support structure is a lattice-like matrix which is obtained bymixing the at least one catalyst powder with dendritic copper in powderform and by compressing this mixture. During compression, the dendriticcopper forms a lattice-like matrix structure in which the catalystgrains are “incorporated”. Even with a relatively low copper powdercontent by mass relative to the total mass of the layer, the dendriticcopper powder can easily be pressed together or sintered to form alattice, has a large surface area and is itself catalytically active.Therefore, the use of dendritic copper powder produces a stabilizing,fixing and heat-distributing lattice in the micrometer range.

The catalyst layer 10 has a relatively large surface area of, forexample, 100 cm². To achieve a more compact structure, the catalystvolume through which the reaction mixture is to flow is spread over aplurality of layers which, however, are arranged not next to oneanother, but rather one behind the other, but in parallel. Anarrangement of this type is illustrated in FIG. 2, which shows a stack20 comprising a large number of catalyst layers 10, 10′ resting on topof one another, the layers which are located at the top in the drawingbeing illustrated spaced apart from one another in order for the way inwhich the catalyst operates to be made clearer.

The catalyst layers 10 have passages 12, 14, 14′ 16 for guiding startingmaterials and products of the catalytic reaction. In the exemplaryembodiment illustrated in FIG. 2, starting-material passages 12 whichrun substantially parallel to the longitudinal edges and form guidepassages which run continuously at right angles to the surface plane ofthe catalyst layer are provided in the catalyst layer, thestarting-material passages 12 of catalyst layers 10, 10′ which lie aboveone another being arranged substantially congruently with respect to oneanother and thus forming a guide passage, which runs continuouslythrough the entire stack 20 from the top downwards, for the startingmaterials of the reaction mixture. Depending on the use of the stackarrangement, a specific reaction mixture is guided through thestarting-material passages 12. If it is being used as a hydrogenreactor, the reaction mixture comprises alcohol, in particular methanol,and chemically bonded hydrogen, advantageously in the form of water. Ifthe stack 20 is being used in a so-called H₂ shift reaction to reducethe levels of carbon monoxide while releasing carbon dioxide, thereaction mixture comprises carbon monoxide and hydrogen. If it is beingused in the carbon monoxide oxidation sector, the reaction mixturecomprises a CO-containing gas and an O₂-containing gas. If the catalyststack 20 is being used in a catalytic burner, the reaction mixturecomprises a combustible starting material and an O₂-containing gas.

The starting-material passages 12 of every second catalyst layer 10 arein communication with distribution passages 14 which run substantiallyparallel to the surface of the catalyst layer 10 and divert at leastpart of the reaction mixture entering through the starting-materialpassages 12 into the interior of the catalyst layer 10.

Consequently, according to the invention, part of the reaction mixturewhich enters through the starting-material passages 12 and is guidedthrough the stack 20, in every second layer plane, is diverted into theinterior of the two adjoining catalyst layers 10, 10′ through thedistribution passages 14, with the result that the catalyst layers whichare arranged above one another are connected in parallel.

In the exemplary embodiment illustrated in FIG. 2, as described twoseparate starting-material passages 12 are provided per catalyst layer10, 10′. This fact can be utilized to supply different substances in thereaction mixture separately from one another, so that individualconstituents of the reaction mixture are only brought together in theplane of the catalyst layer 10.

For this purpose, it is advantageous to employ a catalyst layer having apassage structure such as that which is illustrated in the exemplaryembodiment shown in FIG. 3. The catalyst layer 21 shown in FIG. 3 hasstarting-material passages 22 a, 22 b and product passages 26, thefunction of which in principle corresponds to the starting-materialpassages 12 and product passages 16 described in connection with FIG. 2.A difference from the catalyst layer 10 illustrated in FIG. 2 is thatthe two starting-material passages 22 a, 22 b which are arrangedseparately from one another are not in communication with one anothervia the dispersion passages, but rather the dispersion passages 24 a and24 b which start from each of the starting-material passages 22 a, 22 b,respectively, extend transversely over the catalyst layer 21 but endbefore they reach the opposite starting-material passage 22 b or 22 a.The result is an arrangement of alternately linked passages, which canbe utilized for the separate supply of a (further) gas which is requiredfor or assists with the reaction. If, in the example of the methanolreformer, a mixture of methanol and steam is fed through onestarting-material passage, for example the starting-material passage 22a, oxygen (air) can be supplied through the other starting-materialpassage 22 b. The substances supplied are distributed in the catalystlayer 12 by way of the dispersion passages 24 a, 24 b assigned to thecorresponding starting-material passage and only come into contact withone another in the layer itself. The result is a particularlyhomogeneous and safe (explosion risk) dispersion and mixing of thestarting materials. Of course, embodiments other than those illustrated,with only one starting-material passage or even more than twostarting-material passages, are also possible.

Product passages 16, which are of similar design to thestarting-material passages 12, are arranged along the transverse edgesof the catalyst layers 10, 10′, which product passages likewise formguide passages which run substantially at right angles to the surface ofeach catalyst layer 10 and, when the catalyst layers 10 have been laidon top of one another, are in each case positioned congruently withrespect to the product passages of the catalyst layer 10, 10′ arrangedabove or below. The product passages 16 of each second catalyst layer10′ are in communication with collector passages 14′, which collect thereaction product emerging from the catalyst layer 10, 10′ arranged aboveand below and supply this product in the transverse direction to theproduct passages 16, by means of which the reaction products are removedthrough the stack 20.

In the embodiment illustrated of a device according to the invention forcarrying out a heterogeneously catalysed reaction such as the generationof hydrogen, the catalyst layers 10, 10′ which have been laid on top ofone another therefore have alternating ways of functioning; the startingmaterials which are supplied through the starting-material passages 12are distributed in the catalyst layers 10 and distributed over thesurface of the catalyst layer located above and below by means ofdispersion passages 14, and flow through this layer substantially atright angles and with a considerable pressure drop. In the followingcatalyst layer 10′, the products of the catalytic reaction are collectedin collector passages 14′ and are fed to the product passages 16 inorder for the reaction products to be removed from the catalyst stack20.

Naturally, the invention is not limited to the embodiment illustratedand described. Rather, embodiments in which each catalyst layer isresponsible for supplying, distributing, collecting and removing thestarting materials or products are also conceivable. More complexcatalyst layers of this nature may, for example, be produced by pressingand sintering pulverant catalyst material onto catalyst layers whichhave already been sintered.

Therefore, the invention provides catalyst layers which can be producedeasily and in compact form and are suitable for use in hydrogen reactorsfor the catalytic generation of hydrogen, hydrogen shift stages forreducing the levels of CO, carbon monoxide oxidation reactors andcatalytic burners. The design of the catalyst according to the inventionenables a modular structure to be used, in which there are only lowthermal losses and no temperature gradients, making it possible toachieve a reaction which proceeds homogeneously over a large volume. Theentire volume of the catalyst is physically acceptable to startingmaterials, leading to considerably improved starting dynamics.Furthermore, the risk of ignition in the homogeneous combustion ofmethanol or the hydrogen-oxygen reaction is avoided.

By suitably selecting the process parameters (compression pressure,temperature, type and condition of the starting materials, such asparticle size distribution, porosity, etc.), it is possible for theperson skilled in the art to produce a catalyst layer arrangement orcatalyst layer according to the invention which is tailored to theparticular requirements and is optimized with regard to layer sequence,heat distribution, flow conditions and mechanical properties such aspressure drop and stability.

What is claimed is:
 1. A process for producing a catalyst layer forcarrying out a heterogeneously catalysed reaction, comprising:compressing at least one catalyst that contains dendritic copper inpowder form, thereby forming a compressed layer that forms a shapedbody, wherein, in the compressed layer, the dendritic copper forms alattice support structure for the catalyst material.
 2. A processaccording to claim 1, further comprising sintering the shaped body.
 3. Aprocess according to claim 1, further comprising forming passages in theshaped body during the compressing step for guiding starting materialsand products of the catalysed reaction.
 4. A process according to claim3, wherein said passages are formed by introducing spacer elements intothe shaped body and subsequently removing the spacer elements from theshaped body.
 5. A process according to claim 4, wherein the removing ofthe spacer elements is by burning, pyrolyzing, dissolving, orvaporizing.
 6. Process according to claim 2, further comprising pressinga powder layer onto the sintered shaped body and then sintering thepowder layer.
 7. A process according to claim 1, in which the catalystmaterial comprises a precious metal.
 8. A process according to claim 4,wherein said metal is platinum.